In plasma systems, non-uniformities have been reported in field-free plasmas, that is, in plasmas in which no magnetic field is present. For example, in systems having inductively coupled plasma (ICP) sources, such non-uniformities have been found during plasma assisted etching of large objects greater than 50 cm in length, when 13.56 MHz RF bias was applied. This non-uniformity was reduced at lower bias frequencies, but the cause has not been explained. Uniformity tolerances for integrated circuit manufacturing are becoming more stringent, while wafer size is projected to increase further. Therefore, process uniformity issues are especially important in the processing of large substrates.
Plasma sources used for the processing of semiconductor wafers and some other substrates have evolved from those using simple DC ionization sources to complex RF sources that produce high-density, low energy plasmas. The RF sources include inductively coupled plasma (ICP) sources, and capacitively coupled plasma (CCP) sources. Design of these RF plasmas has traditionally involved the design of antenna and electrode configurations of a unitary source that creates or otherwise introduces bulk plasma in a plasma volume within a processing chamber. Antenna or electrode placement and shape, as well as various chamber properties, are design parameters that are varied to shape the bulk plasma.
The most common inductively coupled plasma (ICP) sources include coils with a planar, cylindrical or dome-shaped geometry. Antennas with more complex shapes have been more recently described in patent literature, including combined, hybrid and dual-coil configurations, coils formed of multiple small solenoids, multiple spirals, multi-zone ICP enhanced PVD coils producing a torroidal plasma, coils designed as a transmission line, embedded coils, planar helicon antennas, 3D antennas, segmented antennas, parallel conductor antennas, etc.
Capacitive coupling plasma (CCP) sources are often driven by a combination of at least two frequencies to achieve independent control of the ion flux and the ion energy impacting the electrodes. This has resulted in a widening of the range of frequencies that are used. A major attraction of dual-frequency excitation is an expectation of the ability to independently control ion flux and the ion energy. Typically in dual frequency systems, a wafer supporting electrode is at lower frequency of, for example, 13.56 MHz, while a second frequency, usually higher than the wafer bias frequency, is provided to give quasi-independent control of the plasma (ion) density and, to some extent, plasma uniformity. In some circumstances, a third frequency is added to further control the etching processes by modifying the ion energy distribution function at the substrate. This multiple frequency excitation produces additional electron heating mechanisms. It has been considered that the electron heating in the sheath is greatly enhanced by the combination of two frequencies, i.e., the heating produced is much larger than the sum of the two single contributions.
With increased size of the wafer and the wafer supporting electrode, electromagnetic effects become important in capacitively coupled plasmas, and a significant amount of heating is provided by the inductive field as the discharge experiences a capacitive-to-inductive transition. Recent work on this subject has focused on two primary points: One group of studies has examined the influence of RF frequency on excitation and ionization changes that impact etch parameters, and provides guidance in the development of advanced plasma sources and processes capable of controlling the plasma and surface chemistry. Another group of studies has focused on non-uniformities introduced into the plasma by the skin effects and standing waves. This group of studies represents potential critical limitations to the practical implementation of high frequency sources for large-area processing.
High-density magnetic-field-free plasma sources produce plasmas that are opaque to radio frequency (RF) fields when using frequencies in the 2-200 MHz frequency range. At these frequencies, a skin effect occurs by which plasma currents from a biased substrate flow to ground along reactor and electrode surfaces. In an ICP source with densities of from 1011 to 1012 cm−3 in argon, an RF skin effect has consequences. Magnetic probe measurements confirm that capacitively coupled RF fields are localized near the reactor surfaces where the skin-effect current flows. Further, RF wavelength and phase velocity along reactor surfaces have been found to be reduced by a factor of about 5 compared to the wavelength in free space. The effective RF wavelength, or wavelength of the applied RF actually measured on the electrode, has been shown to be about one fifth of the RF wavelength at the same frequency in free space. At 200 MHz, the free-space wavelength of 1.5 meters is reduced to an effective wavelength of about 0.3 meters, which is comparable to the dimension of a capacitive electrode or a 300 mm wafer. This can produce differences in voltage or current over the dimensions of a wafer, which can lead to spatial non-uniformities in plasma and plasma processing. Furthermore, the high frequency capacitive discharge can experience capacitive-to-inductive (E to H) transitions when the injected power, i.e. the voltage between the electrodes, is increased. When both the capacitive and inductive power are radially non-uniform, severe problems of process uniformity can result.
Simulations of the RF discharges driven at various frequencies have been published showing that plasma density, ion current and plasma power are proportional to the square of the RF frequency that is driving the discharge, for constant RF voltage. Electron density is also predicted to scale linearly with increased pressure. Charged and neutral particle density has been examined to look for possible effects due to a capacitive-to-inductive transition in the energy deposition mechanism, and to explore CCP operation above the frequencies of 27 and 60 MHz. Multiple frequency excitation was shown to lead to new electron heating mechanisms. The electron heating in the sheath has been shown to be greatly enhanced by the combination of two frequencies, producing heating that is much larger than the sum of the two single contributions. It was observed that the higher frequencies drive the larger density in the central area of a plasma column in CCP, shifting the spatial distribution from one that is uniform to one that is highly peaked in the center. When combinations of two or more frequencies are coupled into the chamber, even at lower than VHF frequencies, products can be produced that have an effective portion in the VHF range sufficient to influence the plasma.
Other approaches to improve non-uniformity in 300 mm systems and large area CCP sources have been described in publications, such as the suppressing of standing wave nonuniformity for large area rectangular CCP reactors. Electromagnetic effects also become important at high frequencies, for which significant heating is provided by the inductive field parallel to the electrode where the discharge experiences a capacitive-to-inductive transition when the high frequency voltage amplitude is raised. The electromagnetic effects of this lead to severe non-uniformity of the power deposition, which, in turn, can ruin process uniformity.
There is a need to solve the problems of increased causes of non-uniformities in the processing of large area substrates.